The invention relates to densifying porous substrates with pyrolytic carbon (PyC) that is deposited within the pores of substrates by chemical vapor infiltration (CVI).
A particular field of application of the invention is making parts out of composite material by densifying porous fiber substrates, in particular substrates made of carbon fibers, with a PyC matrix obtained by chemical vapor infiltration. This produces carbon/carbon (C/C) composite material parts. Because of its thermostructural properties, C/C composite material is suitable for making structural parts that are liable in operation to be exposed to high temperatures, in particular parts for propulsion or structural assemblies in the aerospace field. The friction characteristics of C/C composite materials also make them suitable for constituting friction parts for brakes and clutches, in particular brake disks for airplanes and land vehicles.
The chemical vapor infiltration process is well known. It consists in placing one or more porous substrates for densification inside an oven into which a reaction gas is introduced having at least one component that is a precursor for the material of the matrix to be deposited within the pores of the substrates. The conditions of flow rate, temperature, and pressure are determined so as to enable the gas to diffuse within the pores of the substrates and form therein the desired deposit by one of the components of the gas decomposing or by a plurality of the components of the gas reacting together.
In order to form a PyC matrix, a reaction gas is used that contains one or more gaseous hydrocarbons suitable for producing a carbon deposit by decomposing. A typical example of the reaction gas is a mixture of methane and propane, in which the propane acts as a “dopant” constituting the main source of PyC, while the methane acts essentially as a diluant, encouraging the gas to diffuse into the pores of the substrates, and also providing a fraction of the deposited PyC. The PyC CVI method (the method of depositing a PyC matrix by means of CVI) is generally undertaken at a temperature lying in the range 950° C. to 1100° C., at a pressure of less than 10 kilopascals (kPa).
There exist several PyC CVI processes, and in particular the isothermal method and the temperature gradient method.
In the isothermal process, the substrates for densification are maintained at all times at a temperature that is substantially uniform throughout their volume. A drawback of that process lies in the practical impossibility of achieving densification that is uniform. The matrix material tends to deposit preferentially within the pores that are close to the outside surface of the substrate. Progressive obstruction of the surface pores makes access for the reaction gas to the inside of the substrate more and more difficult, and as a result there is a densification gradient between the surface and the core of the substrate. It is indeed possible to machine the surface or to remove the crust from the substrate one or more times during the densification process in order to open its surface pores. However that requires the process to be interrupted for the time needed to extract the substrate from the densification installation, to cool it, to remove its crust, to reinsert the substrate in the installation, and to return to the desired temperature. The duration of the isothermal PyC CVI process is thus particularly lengthy. Industrially, densifying parts such as C/C composite disk brakes for airplanes using that method commonly requires several hundreds of hours.
With a temperature gradient process, it is possible to a large extent to limit the above-mentioned drawback of the isothermal method. A temperature difference is established within an internal portion of the substrate which is at a higher temperature, and the surface of the substrate which is exposed to the reaction gas. The matrix material then becomes deposited preferentially within the hotter internal portion. By controlling the surface temperature of the substrate so that it remains below the decomposition or reaction threshold of the gas, at least during an initial portion of the densification process, it is possible to ensure that the densification front advances from the inside towards the surface of the substrate as the process continues. In known manner, the temperature gradient can be obtained by placing one or more substrates around a susceptor coupled to an induction coil with an internal face of the substrate(s) in contact with the susceptor. It is also possible to obtain a temperature gradient by direct inductive coupling between the induction coil and the substrate during densification, when the nature of the substrate makes that possible. Those techniques are described in particular in patent documents FR-A-2 711 647 and U.S. Pat. No. 5,348,774.
In document U.S. Pat. No. 5,348,774, the substrates are heated both by coupling with a susceptor and by direct coupling with the substrates as the densification front advances. Means are provided for measuring the variation in substrate weight on a continuous basis so as to monitor how the densification process is progressing. As a function of variation in measured weight, the process can be optimized, in particular concerning its duration, by acting on the parameters of the densification operation, and in particular on the power delivered to the induction coil. Monitoring substrate weight variation can also be used to determine when the end of the densification process has been reached. In comparison with the isothermal method, the temperature gradient method does indeed enable densification to be obtained that is less heterogeneous, however it can be implemented only with substrates of a particular shape, and specifically with substrates that are annular.
Varying densification parameters throughout a CVI process is envisaged in patent document U.S. Pat. No. 6,001,419. That document provides a method of controlling the microstructure of the deposited material. When the material is PyC, it is known that by modifying infiltration conditions it is possible in particular to obtain a pyrocarbon of a smooth laminar type, of a dark laminar type, of a rough laminar type, or of an isotropic type. The microstructure of the pyrocarbon is a characteristic that is important for the properties of the densified substrate. Thus, with carbon/carbon composite material parts, it is often desirable to have a microstructure of the rough laminar type, in particular because of the ease with which it can be turned into graphite by heat treatment.
The method of patent document U.S. Pat. No. 6,001,419 is effective in controlling the microstructure of the deposited PyC, but it also presents the advantage of obtaining a significant reduction in the total duration of the densification process. The densification parameters are varied in accordance with a predefined model.